Why is my heating not working ?

Vaillant 628 thermocompact boiler ( sealed pressurised system)
Drayton 5/2 timer for heating and water + Potterton thermostat + Potterton Motorised Valve.
Hot water heating working ok from timer.
Heating not responding to timer & thermostat.
Thermostat has power, and clicks when manually rotated around sensed actual room temp.
BUT...the boiler motorised valve does not appear to be opening to open the central heating circuit. ( It has a LED showing it's powered)
I can run the boiler in manual for central heating by removing the timer and thermostat as external controls, jumping the terminals in the control panel of the boiler and then manually opening the motorised valve and latching it open.
Where is the problem in the circuit ? Timer, Valve or thermostat ???

steve
February 2009